Highly specific interactions regulate the transmission of many arboviruses by mosquito vectors, influencing their ability to cause disease outbreaks. Emergence of arboviral diseases can also depend on the ability of zoonotic viruses to utilize different mosquito vectors during epidemics and epizootics. Outbreaks can also occur when native mosquitoes transmit newly introduced viruses, or when introduced mosquitoes transmit native viruses. The vector- and virus-specific determinants that regulate all of these arbovirus-mosquito interactions remain poorly understood, particularly at the molecular level. This proposal is designed to elucidate arboviral determinants that regulate the specificity of mosquito transmission, and to assess potential vector switching (adaptation) mechanisms involving these determinants. Venezuelan equine encephalitis (VEE) virus infection of mosquito vectors will be used as a model system because (a) infectious cDNA clones allow for reverse genetic manipulation of these viruses, and (b) dramatic differences in mosquito infectivity have been observed among different VEE serotypes. The specific aims are: 1. Generate chimeric VEE viruses by inserting genome portions of a variety IE isolate, which readily infects Culex (Melanoconion) taeniopus, into an existing infectious cDNA clone derived from a variety IAB strain that inefficiently infects this mosquito. 2. Test chimeric IE/IAB VEE viruses for their ability to infect and be transmitted by Cx. taeniopus, mosquitoes in order to identify viral determinants of vector specificity. 3. Evaluate the ability of two different VEE viruses to adapt to new mosquito vectors. Results generated from these studies will provide the first detailed information on arboviral determinants of vector transmission and mechanisms of mosquito adaptation, increasing understanding of the emergence mechanisms of arboviral diseases, as well as improving strategies for their control.